This invention relates to radio transmission systems in which a message signal is simultaneously transmitted in the same carrier frequency channel from a plurality of radio transmitters.
Such systems are known and are used, for example, in vehicle communication and paging systems in order to cover a large area with improved uniformity of signal strength. Transmitter sites are located so that shadow areas of one transmitter are filled in as far as possible by other transmitters.
Very large areas have been covered by such a system with good results in all locations where a carrier signal received from one of the transmitters is considerably stronger than any signal received from another transmitter. In areas where nearly equal carrier signal levels are received from two transmitters, there can be severe interference, giving loss of speech intelligibility and high error rates in digital data systems. Such areas are hereinafter referred to as overlap areas.
In order to reduce this distortion, very high stability reference oscillators (accuracy of one part in 10.sup.-9) are used at each transmitter site and the delays in the links (land-line or radio) between the originating base station and the various transmitters are equalised. As a result, the carriers transmitted from the transmitters are almost in synchronism--i.e. quasi-synchronous transmission. Further, it is usual to adjust the frequency deviation in FM transmitters, or the depth of modulation in AM transmitters, to be the same for all transmitters. A general description of quasi-synchronous transmission and a discussion of message distortion in overlap areas is given in the article "Quasi-synchronous transmission," Communications International, June 1976, pages 27, 32 and 35.
Even with such precautions, however, the distortion problems still arise when the received carrier levels closely approach each other, for example within 3 dB in FM systems and 6 dB in AM systems. Distortion exists to a greater or lesser degree, irrespective of whether the carriers are frequency- or amplitude-modulated and of whether the message is in the form of speech or a data bit stream. The particular type of distortion experienced does, however, depend upon those factors.
Quasi-synchronous systems are frequently used for data transmission, for example in vehicle identification systems, paging systems, and message transmission. In such systems, the size of the overlap area in which the data signal is not successfully captured is dependent upon the bit rate, i.e. the higher the bit rate the larger the overlap area. It has been shown that, in a large multi aerial system, the overlap areas are very small if the bit rate is in the region of 300 bits per second. In commonly-used systems having carrier frequencies spaced at 25 kHz in the VHF or UHF band concerned, the maximum bit rate is 9600 bits per second; so a very heavy penalty is paid, in terms of the maximum number of messages transmitted per unit of time, in order to reduce the size of the overlap areas. In the case concerned, thirty two times as many messages could be sent if the maximum bit rate of 9600 bits per second could be used.